Sterilization indicators provide a means for determining whether a sterilizing machine, such as those used to sterilize surgical instruments in hospitals, is functioning properly and killing microorganisms that are present in the sterilization chamber during a sterilization procedure.
Biological indicators are recognized in the art as providing an accurate and precise means for testing the effectiveness of a sterilization procedure. Conventional biological indicators gauge the effectiveness of a sterilization procedure by monitoring the survival of a test microorganism contained within the biological indicator that is many times more resistant to the sterilization process than most organisms that would ordinarily be present by natural contamination. The biological indicator is exposed to a sterilization cycle and then incubated under conditions that will promote the growth of any surviving test microorganisms. If the sterilization cycle fails, the biological indicator generates a detectable signal indicating that the biological specimen survived. The detectable signal is commonly an indication such as a color change or the emission of a luminescent or fluorescent signal.
One well-known type of biological indicator employs spores from bacteria or fungi, which are very resistant to sterilization, to test the effectiveness of a sterilization procedure. U.S. Pat. No. 3,661,717 (Nelson) discloses a self-contained biological indicator for monitoring the effectiveness of a sterilization procedure by measuring the survival of a test population of spores. The biological indicator has an outer tube made of a compressible, plastic material and a sealed inner tube made of a breakable material such as glass. A bacteria impermeable, gas transmissive covering on the outer tube allows sterilant to enter the outer tube during a sterilization procedure. Live spores on a piece of carrier material are located between the walls of the outer tube and the inner tube. The inner tube contains a growth medium that stimulates the growth of live spores. During the sterilization procedure sterilant enters the outer tube through the cap and contacts the spores on the carrier strip. After the sterilization procedure, the inner tube is crushed by compressing the outer tube, releasing the growth media and bringing it into contact with the spores on the carrier strip. The indicator is then incubated under conditions that stimulate spore growth. If the sterilization procedure is ineffective, surviving spores will grow out and cause a pH indicator in the growth medium to change color, indicating that the sterilization cycle failed to kill the test population of microorganisms and may have failed to kill contaminating microorganisms present in the sterilizer load. Although biological indicators that rely on the growth of spores are accurate, they are slow, commonly requiring between 1 and 7 days to provide final results. During this incubation period the goods exposed to the sterilization procedure should preferably be quarantined until final indicator results are obtained. However, holding goods in quarantine for such a lengthy period of time requires a substantial commitment of space that could otherwise be used for other purposes, and complicates the efficient regulation of inventory.
Recently sterilization indicators have been developed that measure the effectiveness of a sterilization procedure by measuring the activity of an enzyme whose activity is correlated with the destruction of contaminating microorganisms during a sterilization procedure. Enzyme sterilization indicators are disclosed in U.S. Pat. Nos. 5,252,484 and 5,073,488. In contrast to biological indicators that measure spore growth alone, enzyme indicators provide a rapid answer, often in a matter of a few hours. The indicators have a compressible outer tube, a breakable inner tube, and a cap that is bacteria impermeable but gas trasmissive. Active enzyme is impregnated on a carrier strip located between the walls of the outer and inner containers, and a substrate that reacts with the active enzyme is contained within the sealed inner tube. During the sterilization procedure the sterilant enters the outer tube and contacts the active enzyme on the carrier strip. After the sterilization procedure, the inner vial is crushed and the enzyme strip is exposed to the substrate and incubated. If the sterilization procedure works properly, the enzyme is inactivated during the procedure and there is no detectable change following incubation. However, if the sterilization procedure is ineffective, the enzyme is not inactivated and will react with the substrate to form a detectable product. The enzyme-substrate product may be detectable as a color change or as a fluorescent or luminescent signal.
Dual rapid readout indicators are sterilization indicators that test the effectiveness of a sterilization procedure by measuring both enzyme activity and spore growth following exposure to a sterilization procedure. The enzyme system gives a rapid indication of the effectiveness of a sterilization cycle, which is then confirmed by measurement of spore outgrowth over a longer period of time. In a dual rapid readout indicator, the live spores utilized in the spore outgrowth portion of the indicator may also serve as the source of active enzyme for the enzyme activity portion of the assay. The rapid enzyme test measures the activity of an enzyme that is associated with the spores, and the spores themselves are then incubated to encourage the outgrowth of any spores that survived the sterilization procedure. 3M™ Attest™ 1291 and 1292 Rapid Readout Biological Indicators, available from 3M Company, St. Paul, Minn., are dual rapid readout indicators that test the effectiveness of a sterilization cycle by measuring both the activity of an enzyme associated with Bacillus stearothermophilus spores in the indicator and the survival of the spores themselves.
Although enzyme sterilization indicators are both rapid and accurate for testing the effectiveness of most steam sterilization procedures, the enzyme in an indicator may be prematurely inactivated before all of the contaminating microorganisms have been killed when certain prevacuum, or vacuum assisted, steam sterilization procedures are used. As a result, the sterilization indicator may provide an incorrect indication that the sterilization procedure was effective, or a “false negative” result. The existence of the premature inactivation problem has been detected with dual rapid readout indicators, which after exposure to a suspect sterilization procedure provide a negative result by the enzyme test and a contradictory positive result by the spore growth test. Premature inactivation of enzyme in sterilization indicators has been observed, and is known to be a problem, with 121° C. prevacuum sterilization cycles in which a vacuum is drawn in the sterilization chamber before stream is introduced. Although the precise mechanism underlying this problem is not known with certainty, it is believed that it may be caused by condensed sterilant contacting the enzyme and inactivating it.
Premature inactivation of enzyme has also been observed in dual rapid readout indicators that have been exposed to hydrogen peroxide plasma sterilization procedures. Sterilization processes using hydrogen peroxide plasma are known in the art and are described, for example, in U.S. Pat. No. 4,643,876, issued to Jacobs et al. Plasma refers to the portion of the gas or vapors of a sterilant that includes electrons, ions, free radicals, dissociated atoms and molecules that are produced when an electrical field is applied to the sterilant, and includes any radiation produced by the sterilant after application of the electrical field. In hydrogen peroxide plasma sterilization procedures, a vacuum is typically drawn in a sterilization chamber and hydrogen peroxide vapor is injected and allowed to diffuse throughout the chamber and contact the surfaces of all items that are intended to be sterilized. A vacuum is then drawn to remove the hydrogen peroxide vapor, and a plasma is generated within the chamber by an electrical power source, such as a radio frequency (RF) power source. The power is continued for a period of time sufficient to create a plasma that kills any microorganisms within the chamber. The precise mechanism responsible for premature inactivation in hydrogen peroxide plasma sterilization procedures is not known.
Efforts have been made in the art to prevent premature inactivation of enzyme in sterilization indicators. Commonly assigned U.S. patent application Ser. No. 08/954,218 (Albert) discloses a protective housing that impairs the premature inactivation of enzyme in an enzyme indicator or a dual rapid readout indicator by preventing condensed sterilant from contacting the indicator. The protective housing includes a tube to contain a biological indicator and a cap assembly designed to prevent condensed sterilant from contacting the biological indicator within the tube. The cap assembly includes an aperture through which non-condensed sterilant may enter the housing to contact the biological indicator. An absorbent material within the cap assembly retains condensed sterilant and inhibits fluid from entering the tube and contacting the biological indicator, yet allows noncondensed sterilant to enter the housing. Thus, the Albert application prevents premature inactivation indirectly, by providing a physical barrier that prevents condensed sterilant from contacting the enzyme.
There is a need in the art for an enzyme-based sterilization indicator in which the enzyme is made resistant to premature inactivation by a chemical treatment.